A 100 Mbps LAN that maintains backward compatibility with 10 Base-T networks running at 10
Mbps. Competitor to 100VG AnyLAN.
A 10 Mbps Ethernet LAN which runs over twisted pair wiring. This network interface was
originally designed to run over ordinary twisted pair (phone wiring) but is predominantly
used with Category 3 or 5 cabling.
Modulation format used by ISDN.
One of the 800 series of standards ratified by the IEEE, this specification details MANs
that rely on DQDB, a connectionless packet-switched protocol.
The physical telecommunications circuit connecting an end user location with the serving
central office in a local network environment. Also called the local loop or "last
mile." See also Local Loop.
That portion of a public-switched network that connects access nodes to individual
subscribers. The access network today is predominantly passive twisted pair copper wiring.
Points on the edge of the access network that concentrate individual access lines into a
smaller number of feeder lines. Access nodes may also perform various forms of protocol
conversion. Typical access nodes are DLC systems concentrating individual voice lines to
T-1lines, cellular antenna sites, PBXs, and ONUs.
The transmission speed of the physical access circuit between the end user location and
the local network. This is generally measured in bits per second; also called Access
See Adapter Card.
Circuit board or other hardware that provides the physical interface to a communications
network; an electronics board installed in a computer which provides network communication
capabilities to and from that computer; a card that connects the DTE to the network. Also
called a Network Interface Card. See also Data Termination Equipment and Network Interface
The organization developing and defining xDSL standards, including those affecting ADSL,
SDSL, HDSL, and VDSL. Members participate in committees to vote on ADSL specifications;
auditing members receive marketing and technical documentation.
ADSL Transmission Unit - Central Office
The ADSL modem or line card that physically terminates an ADSL connection at the telephone
service provider's serving central office.
ADSL Transmission Unit - Remote
The ADSL modem or PC card that physically terminates an ADSL connection at the end user's
Current dial-up services require the user to "make a call" to the ISP. The
connection is only active during the duration of the call. Most xDSL implementations
(including ADSL, UADSL, and SDSL) enable the connection to be always on in a fashion
similar to a LAN.
American Wire Gauge
A measure of the thickness of copper, aluminum, and other wiring in the US and elsewhere.
Copper cabling typically varies from 18 to 26 AWG, the higher the number, the thinner the
wire. The thicker the wire, the less susceptible it is to interference. In general, thin
wire cannot carry the same amount of electrical current the same distance that thicker
An electrical signal or wave form in which the amplitude and/or frequency vary
Analog Front End
The analog front ends are responsible for converting the digital signal to analog and
force the signal onto the twisted pair line.
Application Specific Integrated Circuit
A chip designed for a specific application. Examples of an ASIC application can be SDSL or
other broadband solutions.
Asymmetrical Digital Subscriber Line
BellCore term for delivery of digital information over ordinary copper phone lines. ADSL
uses a system of frequency division whereby lower frequency POTS signals are delivered to
the home unaltered while digital signals traverse the phone line at higher frequencies for
delivery to end stations such as a video CODEC or PC. Asymmetric refers to the fact that
the downstream (to the user) channels can outweigh the upstream (to the network) channels
by a ratio as high as 20:1. This asymmetry is a good fit for video on demand and Internet
access applications where the paradigm is a small request up to the network and a large
delivery to the user.
Asynchronous Transfer Mode
A protocol that packs digital information into 53-byte cells (5-byte header and 48 bytes
of payload) that are switched throughout a network over virtual circuits (standardized by
the ITUT in 1988 to create a BISDN). Its ability to accommodate multiple types of media
(voice, video, and data) makes it a likely player for full service networks based on ADSL
Data transmission one character at a time to the receiving device, with intervals of
varying lengths between transmittals and with start bits at the beginning and stop bits at
the end of each character, to control the transmission. In xDSL and in most dial-up modem
communications, asynchronous communications are often found in Internet access and remote
office applications. See Synchronous Transmission.
ATM Adaption Layer 5
A standard adopted by the ATM Forum for a class of service called High Speed Data
An ATM cell is 53 bytes long containing a 5-byte header and a 48-byte payload. The header
of an ATM cell contains all necessary information for data to reach the appropriate
endpoint. The payload portion of an ATM cell can contain any type of information, be it
voice, video, or data.
An ATM connection is actually one physical connection between two endpoints, that contains
multiple VCs. Furthermore, multiple VCs can be grouped to traverse a VP. See also
Permanent Virtual Circuit, Switched Virtual Circuit, Virtual Channel Identifier, and
Virtual Path Identifier.
The organization tasked with developing and defining ATM standards. See
http://www.atmforum.com for more information.
ATM Passive Optical Network
A passive optical network running ATM.
ATM Forum-defined 25.6 Mbps cell-based user interface based on IBM token ring network.
The dissipation of the power of a transmitted signal as it travels over the copper wire
Available Bit Rate
Provides a guaranteed minimum capacity but allows data to be bursted at higher
capacities when the network is free.
The "bearer" channel, in the ISDN scheme. With both basic rate and primary rate
ISDN, this is a full-duplex 64 Kbps channel for sending and receiving data. See also Basic
Rate Interface, Integrated Services Digital Network, and Primary Rate Interface.
That part of a network which is used as the main path for carrying traffic between network
A transmission facility designed to interconnect two or more LAN segments.
A common bus in a chassis that distributes low voltage power to each card slot.
This is a reflection of the size or the capacity of a given transmission channel.
Using the entire bandwidth of a transmission medium, such as to carry a
single digital data signal. Note that this limits such transmission to a single form of
data transmission, since digital signals are not modulated. See also Broadband.
Basic Encoding Rate
Bit error rate, or the ratio of received bits that are in error; also, a rule for encoding
data units described in ANSI. See Bit Error Rate Test.
Basic Rate Access
See Basic Rate Interface.
Basic Rate Interface
This is an ISDN interface typically used by smaller sites and customers. This interface
consists of a single 16 Kbps data (or"D") channel plus two bearer (or
"B") channels for voice and/or data. Also known as Basic Rate Access, or BRA.
Transmission rate of a multilevel-coded system when symbols replace multiple bits. Baud
rate is always less than bit rate in such systems.
A 64 Kbps channel in basic rate and primary rate services of ISDN used for customer
message exchange. See also Basic Rate Interface, Integrated Services Digital Network, and
Primary Rate Interface.
Binary Eight Zero Suppression
A technique in T-1 that modifies the AMI encoding to ensure minimum pulse density without
altering customer data.When eight zeros in a row are detected, a pattern with intentional
bipolar violations is substituted. These violations enable the receiving end to detect the
pattern and replace the zeros.
Cable pairs are typically arranged under the cable sheath in binder groups. The binder is
a spirally wound colored thread or plastic ribbon used to separate and identify cable
pairs by means of color-coding.The enclosed pair group is called a binder group. The
groups are composed of insulated twisted copper pairs that are also twisted within each
binder.Typically they are wrapped in 25 pair bundles. For example, pairs 1-25 might be in
one binder group and pairs 26-50 in another. In xDSL, one often hears discussions of
signal interference between adjacent pairs within a binder group. The best of all worlds
is to keep a data pair separated from another data pair by assigning it to an adjacent
bindergroup. If the data pairs are too close to each other they create what telcos call
"disturbers" (i.e., crosstalk). If a "disturber" exists in the binder
group serving your SNI, NID, MPOE, etc., you may not "qualify" for DSL service.
Bipolar Return to Zero
A bipolar signal in which each pulse returns to zero amplitude before its time period
ends. This prevents the buildup of DC current on the signal line.
The occurrence of two successive pulses of the same polarity in a bipolar signal. In T-1
it is detected as an error.
A contraction of "binary digit." A bit is the smallest element of information in
the digital system.
Bit Error Rate Test
A test that reflects the ratio of errored bits to the total number transmitted. Usually
shown in exponential form (10^6) to indicate that one out of a certain number of bits are
A technique in T-1 multiplexing in which the least significant bit (bit 8) of each byte in
selected frames is robbed from being used to carry message information and instead is used
to carry signaling information.
Bits Per Second
The raw data rate of a system.
A sometimes-accidental connection of another local loop to the primary local loop.
Generally it behaves as an open circuit at DC, but becomes a transmission line stub with
adverse effects at high frequency.It is generally harmful to DSL connections and should be
removed. Extra phone wiring within one's house is a combination of short bridge taps. A
POTS splitter isolates the house wiring and provides a direct path for the DSL signal to
pass unimpaired to the ATU-R modem.
Sharing the bandwidth of a medium such as copper or fiber optic cable, to carry more than
one signal -- allowing the integration of voice, video, and data over a single
transmission medium; VDSL is a broadband transmission scheme, for instance.
Broadband Integrated Services Digital
A technology suite designed for high-bandwidth multimedia applications and the integration
ofvoice, data, and video. The two transmission types are ATM and STM.
A device that can provide the functions of a bridge, router, or both concurrently; a
brouter can route one or more protocols, such asTCP/IP and/or XNS, and bridge all other
A bus network is a multiple access medium for small networks and usually only consists of
one cable and the devices that are attached to it.
In the telephone network, multiple insulated copper pairs are bundled together into a
cable called a cable binder.
A modem designed to operate over cable TV lines; used to achieve extremely fast access to
Campus Area Network
A network that involves interconnectivity between buildings in a set geographic area, such
as a campus, an industrial park,or other such private environment.
Carrier Service Area
Area served by a LEC, RBOC, or telco, often using DLC technology.
Carrierless Amplitude and Phase Modulation
Licensed by GlobeSpanTechnologies, Inc., this modulation is based on QAM and is used in
ADSL modems. CAP is a competitor to DMT modulation. Regardless of its standards status, it
is used in many telephone companies' ADSL trials.
Category 3 Cabling
A rating for twisted pair copper cabling that is tested to handle 16 MegaHertz of
communications. Handles 10 Mbps of LAN traffic and is commonly used as telephone wiring.
Category 5 Cabling
A rating for twisted pair copper cabling that is tested to handle 100 MegaHertz of
communications. CAT-5 cable is generally required for higher-speed data communications,
such as Ethernet LANs and possibly low-speed ATM.
A circuit switch that terminates all the local access lines in a particular geographic
serving area; a physical building where the local switching equipment is found. xDSL lines
running from a subscriber's home connect at their serving central office. See also Serving
A generic term for a communications path on a given medium; multiplexing techniques allow
providers to put multiple channels over a single medium. See also Multiplexer.
Channel Service Unit/Data Service Unit
A digital interface device that connects end user data communications equipment to the
digital local access lines.
Circuit Switched Network
A network that establishes a physical circuit temporarily on demand (typically when a
telephone or other connected device goes off hook) and keeps that circuit reserved for the
user until it receives a disconnect signal.
A switching system that establishes a dedicated physical communications connection between
endpoints, through the network, for the duration of the communications session; this is
most often contrasted with packet switching in data communications transmissions. See also
A distributed computing model that involves distributing information resources in servers
that are accessed by so-called clients, the end users. This is generally opposed to
centralized mainframe computing architecture.
A hardware device or software program that converts analog information streams into
digital signals, and vice versa; generally used in audio and video communications where
compression and other functions may be necessary and provided by the CODEC as well.
Commercial End User
See Service User.
Community Antenna Television
Also known as Cable TV.
Competitive Access Provider
See Carrierless Amplitude and Phase Modulation.
Competitive Local Exchange Carrier
A LEC which, when competition begins, has the less dominant position in the market; the
carrier entering the market, challenging the ILEC market position.
The act of sampling and reducing a signal for the purposes of saving storage or
transmission capacity; MPEG1 and MPEG2 are two key encoding and compression algorithms
that enable full-motion video over smaller bandwidth circuits, such as those offered by
ADSL, SDSL, and HDSL.
A device that serves as a point of consolidation of network links so that multiple
circuits may share common limited network resources.
A term applied to network architecture and services which require the establishment of an
end-to-end, predefined circuit prior to the start of a communications session. Frame relay
circuits are examples of connection-oriented sessions. See Connectionless.
A term applied to network architecture and services which do not involve the establishment
of an end-to-end, predefined circuit prior to the start of a communications session. Cells
or packets are sent into the connectionless network, and are sent to their destination
based on addresses contained within their headers. The Internet and SMDS are two examples
of connection-less networks. See Connection Oriented.
Constant Bit Rate
Specifies a fixed bit rate so that data is sent in a steady stream. This is analogous to a
Consultative Committee for International
Telegraph and Telephone
Thef ormer international standards body that developed telecommunications standards; now
called the International Telecommunications UnionTelecommunications Standards Sector
Combination of switching offices and transmission plants connecting switching offices
together. In the US, local exchange CoreNetworks are linked by several competing
Interexchange networks; in the rest of the world, the Core Network extends to national
Interference on an analog line of an adjacent signal with the intended receive signal.
Crosstalk makes it hard to hear just the intended signal, as there are multiple
conversations on the line at once.
Customer Premises Equipment
A wide range of customer premises-terminating equipment which is connected to the local
telecommunications network. This includes telephones, modems, terminals,routers, set top
Cyclic Redundancy Check
A test used to confirm that data has been delivered without error. In a data packet, the
CRC character is calculated by assigning binary values to blocks of data. If the block of
data does not match its assigned CRC value upon delivery, the data is errored.
The "data" signaling channel, in the ISDN scheme; this is the channel that
carries signaling and limited packet communications. In basic rate ISDN, this is a
full-duplex 16 Kbps channel; in primary rate ISDN, this is a full-duplex 64 Kbps channel.
See Basic Rate Interface, Integrated Services Digital Network, and Primary Rate Interface.
Data Circuit Terminating Equipment
Any device that is connected to the subscriber end of a transmission circuit and provides
the appropriate termination functions for that connection. A modem or DSU/CSU are
considered DCE. Also called Data Communications Equipment.
Data Link Connection Identifier
The frame relay virtual circuit number used in internetworking to denote the port to which
the destination LAN is attached.
Data Service Unit
A digital interface device that connects end user data communications equipment to the
digital access lines and provides framing of sub-64 Kbps customer access channels onto
higher-rate data circuits. ADSU may be combined with a CSU into a single device called a
CSU/DSU. See Channel Service Unit/Data Service Unit.
Data Terminal Equipment
Typically the device that transmits data such as a personal computer or data terminal.
A transmission circuit that is reserved by the provider for the full-time use of the
subscriber. Also called a Private Line.
Conversion of a carrier signal or wave form (analog) into an electrical signal (digital).
Dense Wave Division Multiplexing
A SONET term. High-speed versions of WDM, which is a means of increasing the capacity of
SONET fiber optic transmission systems through the multiplexing of multiple wavelengths of
light. Each wavelength channel typically supports OC-48 transmission at 2.5 Gbps. A
32-channel system will support an aggregate 80 Gbps.
The process of initiating a switched connection through the network; when used as an
adjective, this is a type of communication that is established by a switched-circuit
Digital Access and Cross-Connect System
A digital cross-connect device for routing lines among multiple ports.
Digital Data Service
Private line digital service with data rates of 56/64 Kbps.
Digital Equipment Corporation Network
Digital Equipment Corporation's proprietary network architecture.
The progression of digital transmission standards typically starting with DS-0 (64 Kbps)
and going up through at least DS-3. Twenty-four DS-0s make up a DS-1; 28 DS-1s make up a
DS-3. There are other links (including a DS-2), but these are less common.
Digital Loop Carrier
Equipment used to concentrate many local loop pairs onto a few high-speed digital pairs or
one fiber optic pair for transport back to the CO.
Digital Signal 0
In the digital hierarchy, this signaling standard defines a transmission speed of 64 Kbps.
Digital Signal 1
In the digital hierarchy, this signaling standard defines a transmission speed of 1.544
Mbps; a DS-1 is composed of 24 DS-0signals; this term is often used interchangeably with
Digital Signal 3
In the digital hierarchy, this signaling standard defines a transmission speed of 44.736
Mbps; a DS-3 is composed of 28 DS-1signals; this term is often used interchangeably with
Digital Signal Processor
A specialized computer chip designed to perform speedy, complex operations on digitized
Digital Subscriber Line
A general term for any local network loop that's digital in nature; technically, DSL
equates to ISDN, but this is decreasingly enforced terminology. See Asymmetrical DSL, High
Bit Rate DSL, Symmetric DSL, Very High Data Rate DSL, and xDSL.
Digital Subscriber Line Access Multiplexer
A CO device used to aggregate data traffic from many DSL subscribers into one high-speed
signal for hand-off to the data communications network.
Discrete Wavelet Multi-Tone
A multicarrier modulation system pioneered by Aware Inc. that, according to the vendor,
isolates its subchannels in a method that is superior to conventional DMT modulation. In
the vendor's own words, "DWMT is able to maintain near optimum throughput in the
narrowband noise environments typical of ADSL, VDSL, and Hybrid Fiber Coax, while DMT
systems may be catastrophically impaired."
Portion of the telephone cabling plant that connects subscribers to feeder cables from the
In xDSL, the communications from the network towards the customer premises.
Basic North American 64 Kbps digitized voice channel.
First level in North American digital hierarchy; the 1.544 Mbps signal consists of 24
DS-0s multiplexed together.
Dynamic Host Control Protocol
A TCP/IP protocol.
Dynamic Random Access Memory
Memory used to store data in PCs and otherdevices.
The European equivalent of a T-1 circuit. It is a term for a digital facility used for
transmitting data over a telephone network at 2.048 Mbps.
The European equivalent of a T-3 circuit. It is a term for a digital facility used for
transmitting data over a telephone network at 34 Mbps.
Echo Suppressor/Echo Canceller
These are active devices used by the phone company to suppress positive feedback (singing)
on the phone network.They work by predicting and subtracting a locally generated replica
of the echo based on the signal propagating in the forward direction. Modems deactivate
these devices by sending the 2100 Hz answer tone with 180-phase reversals every 450 msec
at the beginning of the connection.
Association/Telecommunications Industry Association
This organization provides standards for the data communications industry to ensure
uniformity of the interface between DTEs and DCEs.
Element Management System
A management system that provides functions at the element management layer.
A widely dispersed, multifaceted telecommunications network for a particular purpose or
organization; a term for all of an organization's telecommunications networking services
A LAN used to connect devices within a single building or campus at speeds up to 10 Mbps.
Within the OSI model, Ethernet is defined at layer one (physical) and layer two (data
link). Based on Carrier Sense Multiple Access/Collision Detection (CSMA/CD), Ethernet
works by simply checking the wire before sending data. Sometimes two stations send at
precisely the same time in which case a collision is detected and retransmission is
attempted. See also 10Base-T.
Far End CrossTalk
Leakage of one or more foreign sources into the receiver of a system at the distant end of
a transmission system.
A LAN used to connect devices within a single building or campus at speeds up to 100 Mbps.
Within the OSI model, Fast Ethernet is defined at layer one (physical) and layer two (data
link). Like Ethernet, it uses CSMA/CD.
Federal Communications Commission
The US federal regulatory agency responsible for regulating interstate and international
That portion of the telephone cable plant that extends from the CO to distribution frames
where distribution cables deliver traffic to subscribers.
Feeder Distribution Interfaces
Points where cable bundles from the telephone switch use drop lines extended out to
Fiber Distributed Data Interface
A LAN token ring standard using fiber optic cable.
Fiber Optic Cable
A transmission medium composed of glass or plastic fibers; pulses of light are emitted
from a laser-type source. Fiber optic cabling is the present cabling of choice for all
interexchange networks,and increasingly for the local exchange loops as well; it is high
security, high bandwidth, and takes little conduit space. Considered the physical medium
of all future, land-based communications.
Fiber To The Cabinet
Network architecture where an optical fiber connects the telephone switch to a street-side
cabinet where the signal is converted to feed the subscriber over a twisted copper pair.
Fiber To The Curb
A telephone company service delivery system that delivers voice and video programming to
small clusters of residences using fiber optics as the feeder and either twisted pairs or
coax cable as the distribution plant to each home.
Fiber To The Home
Network where an optical fiber runs from telephone switch to the subscriber's premises or
File Transfer Protocol
Internet tool for accessing files linked to the Internet.
A high-speed, packet-switched data communications service,similar to X.25. Frame relay is
a leading contender for LAN-to-LAN interconnect services, and is well suited to the bursty
demands of LAN environments. See also Permanent Virtual Circuit and Switched Virtual
Frequency Division Multiplexing
A multiplexing technique that uses different frequencies to combine multiple streams of
data for transmission over a communications medium. FDM assigns a discrete carrier
frequency to each data stream and then combines many modulated carrier frequencies for
Refers to the transmission of data in two directions simultaneously.
Gigabits Per Second
A measure of bandwidth capacity or transmission speed. It stands for a billion bits per
Graphical User Interface
A program interface that takes advantage of the computer's graphics capabilities to make
the program easier to use.
HDSL Transmission Unit - Central Office
The HDSL modem or line card that physically terminates an HDSL connection at the telephone
service provider's serving central office.
HDSL Transmission Unit - Remote
The HDSL modem or PC card that physically terminates an HDSL connection at the end user's
The basic unit of frequency measurement; one cycle per second.
High Bit Rate Digital Subscriber Line
Modulation method that enables T-1and E-1 signals to be delivered over two and three pairs
of copper wire respectively. Originally designed to bypass costly repeater installation
required to provision T-1 and E-1 services to the far flung, HDSL is now being positioned
in single-pair configurations that will deliver up to 768Kbps to residences.
High Density Bipolar-3
A bipolar coding method that does not allow more than three consecutive zeros.
High Pass Filter
A signal filter which would be installed in a customer premises ADSL modem (ATU-R), which
only allows higher frequency data to be delivered to the modem. See Low Pass Filter.
Hybrid Fiber Coax
A cable TV delivery system that uses fiber optics to feed a distribution node that
delivers video traffic over coax cable to about 500 homes.
Hyper-Text Markup Language
An authoring software used on the Internet's World Wide Web. HTML is basically ASCII text
surrounded by HTML commands.
Incumbent Local Exchange Carrier
A LEC which, when competition begins, has the dominant position in the market; the
original carrier in the market prior to the entry of competition.
Industry Standard Architecture
A personal computer bus architecture.
Institute of Electrical and Electronic
A professional group that designs and defines network standards, LAN standards in
Insulation Displacement Connection
A type of wire connection device in which a wire is punched down into a metal holder that
strips away the insulation to achieve electrical connection.
Integrated Services Digital Network
Circuit-switched digital network and, technically, the first digital subscriber line.
Comes in two varieties: basic rate interface which runs at 144 Kbps and primary rate
interface which runs at 1.544 Mbps. See also Basic Rate Interface, Broadband ISDN, and
Primary Rate Interface.
Very much a US term, an IXC is a long distance telecommunications provider that offers a
range of circuit-switched, packet-switched, leased line, and enhanced communications
services; any company that provides communications services between exchanges on a long
haul basis. In Europe, Asia, and other nations around the world, the local telco also
serves as the major IXC in the country. See Telco.
A point of connection between two systems, networks, or devices.
Telecommunications services that originate in one and terminate in another LATA.
International Standards Organization
International organization for standardization which is based in Geneva. Publishes
national and international standards for data communications. US representative to ISOis
UnionTelecommunications Standards Sector
Formerly the CCITT, the major international standards body governing telecommunications;
it is a committee of the ITU, a United Nations treaty organization.
World's largest computer network; sprung out of a research effort initiated by US
government. Initially used to connect defense contractors and US Universities. Today its
nature is more commercial and is becoming the preferred method of linking businesses' and
individuals' computers to one another.
The physical telephone circuit connection between the subscriber and the nearest Internet
Internet Access Node
The Internet access provider's facility for receiving communications from subscribers and
prepping it for transmission into the Internet. An ISP access node typically consists of
analog modem, ISDN, and soon xDSL access multiplexers to accept the public network
subscriber connections; routers to packetize the communications into TCP/IP; and frame
relay switches to serve as the fast-packet connection into the Internet. See Transmission
Control Protocol/Internet Protocol.
Internet Engineering Task Force
The primary working body developing new TCP/IP standards for the Internet.
A standard describing software that keeps track of the Internetwork addresses for
different nodes, routes outgoing messages, and recognizes incoming messages.
Internet Service Provider
The telecommunications company providing subscriber access into the Internet.
Internetwork Packet Exchange
LAN communications protocol used to move data between server and workstation programs
running on different network nodes.
The ability of equipment from multiple vendors to communicate using standardized
Telecommunications services that originate and terminate inthe same LATA.
ISDN Digital Subscriber Line
Uses ISDN transmission technology to deliver data at 128 Kbps into an IDSL "modem
bank" connected to a router.
Interference on an analog line caused by a variation of a signal from its reference timing
slots. Jitter can cause problems in the receipt of data.
Joint Photographic Experts Group
A media industry group designing and developing standards for the transmission of images
over various media and networks.
Kilobits per second
A measure of bandwidth capacity or transmission speed. It stands for a thousand bits per
One thousand hertz.
Typically used in LANE over ATM.
Refers to the local loop and is the difference between a local telephone company office
and the service user; a distance of about two to three miles or three to four kilometers.
A measure of the temporal delay. Typically, in xDSL, latency refers to the delay in time
between the sending of a unit of data at one end of a connection until the receipt of that
unit at the destination.
OSI reference model; each layer performs certain tasks to move the information from the
sender to the receiver. Protocols within the layers define the tasks for the networks but
not how the tasks are accomplished.
Layer Two Tunneling Protocol
An extension to the PPP protocol that enables ISPs to operate VPNs. L2TP merges the best
features of two other tunneling protocols: PPTP and L2F.
A telecommunications transmission circuit that is reserved by a communications provider
for the private use of a customer. Also called a private line or nailed up circuit.
A twisted pair phone line with inductors, or loading coils, inserted periodically to
flatten the frequency response in the 4 KHz voiceband.
A device used to extend the range of a local loop for voice grade communications. They are
inductors added in series with the phoneline which compensate for the parallel capacitance
of the line. They benefit the frequencies in the high end of the voice spectrum at the
expense of the frequencies above 3.6 KHz. Thus, loading coils prevent DSL connections.
Local Access Transport Area
The US term LATA arose out of the post-divestiture fight between the local telephone
companies and AT&T over who could carry which traffic as AT&T split itself up.
Roughly, a LATA maybe geographically defined as larger than a local calling area and
smaller than a whole state. However, this is not a hard and fast rule -- the state of
Connecticut has one LATA, for instance. The importance of the LATA is that it defines the
operational areas of the US telco until it is allowed to venture beyond that boundary per
the US 1996 Telecommunications Act provisions for the release of the telcos into the long
Local Area Network
A data communications network covering a small area, usually within the confines of a
building or floors within a building; a relatively high-speed computer communications
network for in-building data transfer and applications. Common LAN protocols are Ethernet
and Token Ring. See also Campus Area Network, Metropolitan Area Network, and Wide Area
Local Exchange Carrier
One of the US telephone access and service providers that resulted from the US
deregulation of telecommunications.
A generic term for the connection between the customer's premises (home, office, whatever)
and the provider's serving central office. Historically, this has been a wireline
connection; however, wireless options are increasingly available for local loop capacity.
Also colloquially referred to as "the last mile" (even though the actual
distance can vary).
Local Multipoint Distributed Service
A terrestrial wireless broadcasting service, principally intended for video distribution,
that has been proposed by the FCC for operation in the band which operates in the same
band as FSS uplink and feeder link transmissions (28 GHz). This band can supply two-way
communications, meaning it could be used for interactive TV, data, and telephone services.
Representing the communications of information over a distance other than the local
calling area. Also called "long haul" traffic.
A term for long distance.
Portion of the telephone network that connects the subscriber to the CO. See Local Loop.
Any tests in which a test signal is injected at one end of a circuit, is looped back at
the other end, and monitored at the originating end.
Low Pass Filter
A signal filter installed in a customer premises ADSL modem (ATUR), which would not modify
the low frequencies present in its input signal (the POTS transmission is sent unmodified
to a phone), but would prevent the high-frequency components (data) from reaching a
customer's telephone. See High Pass Filter.
A piece of telecommunications equipment which multiplexes (combines )28 DS-1 signals into
a single DS-3 signal, commonly used for concentrating traffic for economies of
transmission. See also Multiplexer.
Main Distribution Frame
Central point where all local loops terminate in the CO.
Management Information Base
A database of objects that can be monitored by a network management system. Both SNMP and
RMON use standardized MIB formats that allow any SNMP and RMON tools to monitor any device
defined byan MIB.
Mean Time Between Failure
MTBF ratings are measured in hours and indicate the sturdiness of hard disk drives and
printers. Typical disk drives for personal computers have MTBF ratings of about 300,000
hours.This means that of all the drives tested, one failure occurred every 300,000 hours
of testing. However, this is rather meaningless since most disk drives are tested only a
few hours, so it would be unlikely for a failure to occur during this short testing
period. A more useful gauge of a device's lifetime is how long a warranty the manufacturer
Media Access Control
A method of controlling access to a transmission medium (e.g., Ethernet, Token Ring, and
Megabits Per Second
A measure of bandwidth capacity or transmission speed. It stands for a million bits per
Metropolitan Area Ethernet
A MAE is a NAP where ISPs can connect with each other. The original MAE was set up by a
company called MFS and is based in Washington, DC. Later, MFS built another one in Silicon
Valley, dubbed MAE West. In addition to the MAEs from MFS, there are many other
NAPs. Although MAE refers really only to the NAPs from MFS, the two terms are often used
Metropolitan Area Network
A data communication network typically covering the geographic area of a city; a
communications network that is usually larger than a LAN but smaller than a WAN. See
Campus Area Network, Local Area Network, and Wide Area Network.
Metropolitan Serving Area
A regional area served by a provider which is classified based on the metropolitan
Minimum Point Of Entry
The closest practical point to where the carrier facilities cross the property line or the
closest practical point to where the carrier cabling enters a multiunit building or
Mobile Telephone Switching Office
A generic name for the main cellular switching center which supports multiple base
A term which is actually based on the function of the device itself -- a
MOdulator/DEModulator. A modem converts analog waveforms into digital data and vice versa.
In xDSL, the device that is at each end of the xDSL circuit is being generically referred
to as a modem. See also Demodulation and Modulation.
Moderate Speed Digital Subscriber Line
A slower variant of HDSL, MDSL is widely used in pairgain systems to increase the
effective capacity of a copper loop.
The process whereby an electrical carrier wave is altered to facilitate the transmission
of a signal.
Motion Picture Experts Group
This is an industry organization whose goalis to develop standards and specifications for
the encoding, transmission,and unencoding of video information over various media and
network technologies. See also MPEG1 and MPEG2.
Movies on Demand
See Video on Demand.
Compression scheme for full motion video. Compression algorithm introduced by MPEG in
1991, the common goal of MPEG is to compress 7.7 Meg down to about 150 Kbytes. MPEG1 is
designed to provide a resolution of 352 by 240 pixels at 30 frames per second.
Video compression standard selected by MPEG. Designed to provide a resolution of 720 by
480 pixels at 30 frames per second. Likely to be the leading compression algorithm for a
range of video applications including video on demand.
Multichannel Multipoint Distributed Service
Also known as wireless cable. MMDS is a pay television delivery system that delivers up to
33 channels of video programming via microwave transmission. MMDS systems consist of four
parts (1) a head end, located a top a tall building, where broadcast and cable TV signals
are received and converted to microwave radio signals for retransmission. They are sent
using (2) an omnidirectional transmit antenna to subscribers who are equipped with
(3)receiving antennas which convert the microwave frequencies to lower frequencies, and
send them to (4) a TV or VCR. MMDS is line of sight transmission; the receiving antenna
must have unobstructed view of the transmitting antenna. MMDS systems operate a band of
radio spectrum that ranges from 2.5 billion cycles a second to 7 billion. That band can
only be used for broadcast (one-way) communications.
Multiple System Operator
A company that operates more than one cable TV system.
A MUX is a device that allows the transmission of multiple data streams over a common
medium. Several communications paths or channels may be either permanently or dynamically
established over the medium to accomplish this. See also Channel.
The process of dedicating a telecommunications circuit for a particular use; the physical
or logical dedication of a line for a particular use. See also Leased Line.
A term used to describe services with up to and including T-1or 1.544 Mbps.
Same as ISDN.
Near End CrossTalk
Leakage of undesired local signals into the local retriever; could be from the companion
transmitter or other nearby sources.
Network Access Point
A NAP is a public network exchange facility where ISPs can connect with one another in
peering arrangements. The NAPs are akey component of the Internet backbone because the
connections within them determine how traffic is routed. They are also the points of most
Network Access Provider
Another name for the provider of networked telephone and associated services, usually in
Network Address Translation
The translation of an Internet Protocol address (IP address) used within one network to a
different IP address known within another network. One network is designated the inside
network and the other is the outside. Typically, a company maps its local inside network
addresses to one or more global outside IP addresses and unmaps the global IP addresses on
incoming packets back into local IP addresses. This helps ensure security since each
outgoing or incoming request must go through a translation process that also offers the
opportunity to qualify or authenticate the request or match it to a previous request. NAT
also conserves on the number of global IP addresses that a company needs and it lets the
company use a single IP address in its communication with the world.
Network Design Interface Specification
Used for all communication with network adapters. Works primarily with LAN manager and
allows multiple protocol stacks to share a single NIC.
Network Interface Card
The circuit board or other form of computer hardware which serves as the interface between
a computer, or other form of communicating DTE, and the communications network; in ADSL, a
common NIC card is an Ethernet NIC card which serves as the interface to the ADSL modem
from the computer. See also Adapter.
Network Interface Device
A device that terminates copper pair from the serving central office at the user's
destination and which is typically located outside that location.
Network Interface Unit
Same as a Network Interface Device (NID).
Network Management System
That system which allows a provider or enduser to manage portions or all of a
telecommunications network; in xDSL, network management systems allow providers to control
and monitor those services based on the ADSL streams, at both the physical and logical
layers of the services.
Network Service Provider
The term for an organization offering and providing value-added network services on a
Network Termination Equipment
The equipment at the end of the line.
Network Termination Unit
Equipment at the customer premises which terminates a network access interface.
A node on a network is usually formed by the presence of a router and user access
equipment. Often, several leased lines are joined together at a network node.
Describes a contiguous bit stream at the Nx64 Kbps rate to an application. Examples are
LAN interconnect and point-to-point videoconferencing.
Open Systems Interconnection
A seven-layer architecture model for communications systems; the OSI model was created by
the International Standards Organization (ISO).
Operations, Administration, and Maintenance
A group of network management functions that proved network fault notification,
performance information, and diagnosis functions.
Base unit found in the SONET hierarchy; the "x"represents increments of 51.84
Mbps (so, OC-1 is 51.84 Mbps, OC-3 is 155Mbps, and OC-12 is 622 Mbps). See also
Synchronous Optical Network.
Optical Network Unit
A form of access node that converts optical signals transmitted via fiber to electrical
signals that can be transmitted via coaxial cable or twisted pair copper writing to
A sub-unit of a data stream; a grouping of information that includes a header (containing
information like address destination) and, in most cases, user data.
Packet Switched Network
Network that does not establish a dedicated path through the network for the duration of a
session, opting instead to transmit data in units called packets in a connectionless
manner. Datastreams are broken into packets at the front end of a transmission, sent over
the best available network connection, and then reassembled in their original order at the
A switching system that uses a physical communications connection only long enough to
transit a data message; data messages are disassembled into packets and reassembled at the
receiving end of the communication link. Packets may travel over many diverse
communications links to get to the common endpoint. This is most often contrasted with
circuit switching in data communications, where all data messages transmitted during a
session are transmitted over the same path for the duration of the session. See also
A range of frequencies that has a non-zero lower limit and some upper limit.
Passive Optical Network
A fiber-based transmission network containing no active electronics.
In the cable television industry, the ability to view programming on a pay-per-play basis.
See also Video on Demand.
That portion of a frame or cell that carries user traffic. It is effectively what remains
in the frame or cell if you take out all headers or trailers.
Permanent Virtual Circuit
A term found in frame relay and ATM networkingi n which a virtual connection between two
fixed endpoints is established through the network. See also Switched Virtual Circuit.
Plain Old Telephone Service
This term commonly refers to standard telephony, as in placing and receiving telephone
Point Of Presence
A POP refers to a node of an ISP or other NSP. A POP is usually a network node.
A common, layer two protocol used with Internet protocols and services.
Point-to-Point Tunneling Protocol
A new technology for creating VPNs, developed jointly by Microsoft Corporation, US
Robotics, and several remote access vendor companies, known collectively as the PPTP
Forum. A VPN is a private network of computers that uses the public Internet to
connect nodes. Because the Internet is essentially an open network, PPTP is used to ensure
that messages transmitted from one VPN node to another are secure. With PPTP, users can
dial in to their corporate network via the Internet.
Postal Telephone and Telegraph
Generic term for a provider of access services. A governmental agency in many countries.
A device that separates an xDSL communications stream into its POTS and data components,
offering a standard POTS connection such as an RJ-11 jack in the US, and an Ethernet or
ATM connection, such as an RJ-45 jack. See also Low Pass Filter, High Pass Filter.
Primary Rate Interface
This is an ISDN interface typically used by larger customers. This interface consists of a
single 64 Kbps data (or"D") channel plus 23 or 30 bearer (or "B")
channels for voice and/or data. Also known as Primary Rate Access, or PRA.
See Dedicated Line.
Protocol Data Unit
A segment of data generated by a specific layer in a protocol stack; usually consists of a
block of data from a higher layer (the Service Data Unit or SDU) encapsulated by the next
lower layer with a header and trailer.
Private Branch Exchange
A private telephone network used within an enterprise. Users of the PBX share a certain
number of outside lines for making telephone calls external to the PBX. Most medium-sized
and larger companies use a PBX because it's much less expensive than connecting an
external telephone line to every telephone in the organization. In addition, it's easier
to call someone within a PBX because the number you need to dial is typically just three
or four digits.
Public Utilities Commission
Each state's regulatory agency responsible for regulating intrastate communications.
Quadrature Amplitude Modulation
A modulation technique used by modems and DSL equipment in which a carrier's amplitude and
phase are simultaneously modulated by the digital traffic.
Radio Frequency Interference
Radio frequency is the electromagnetic spectrum from 3 KHz to 300 GHz.
Rate Adaptive Digital Subscriber Line
ADSL modems that are able to adjust to varying lengths and qualities of lines are said to
be rate adaptive. Unlike fixed rate ADSL modems, these modems will connect over varying
lines at varying speeds, making them a good choice for service providers attempting to
deploy ADSL past 18,000 feet. Modems can be designed to select their connection speed at
train up, during a connection, or upon signal from the central office.
A forward error correcting code that is used to offset the effects of bit error bursts in
the receive-bit stream.
Regional Bell Operating Company
An all-inclusive term for each of the seven telephone companies which were created after
Local loop terminates at remote terminal intermediate points closer to the service user to
improve service reliability.
Remote Termination Unit
A device installed at the service user site that connects to the local loop to provide
high-speed connectivity. Also referred to as the ATU-R.
Resource Reservation Protocol
A reservation setup protocol for the Internet.
A term for a DSL stream that is asymmetrical in the upstream direction; that is, a reverse
ADSL link has a small downstream and large upstream communications path.
Six-conductor modular jack used with four-wire cabling. Most common phone jack in the
world and is used commonly on phones, modems, and fax machines.
Eight-pin connector used to attach data transmission devices to standard telephone wiring.
Commonly used in 10Base-T connections.
The device that connects multiple computer networks by reading layer three addressing on
incoming and outgoing packets. Packet information is read and the packets are then
forwarded to the appropriate end station.
Usually made from either silicon or germanium, semiconductors make it possible to
miniaturize electronic components, such as transistors, which require less space and are
faster and more energy-efficient.
Serial Line Internet Protocol
An older layer two protocol used for Internet traffic; much less sophisticated than PPP.
Service Level Agreement
Outlines minimum acceptable performance parameters (such as delay, throughput, percent
availability, etc.) for public data services such as frame relay, VPNs, DSL access, and
The end user at the customer premises.
Serving Central Office
The central office in the local communications network that is directly connected to the
end user location in question.See Central Office.
Set Top Box
A transmission/reception device that acts as an interface typically to a television or
other video output display device. In addition to ADSL, SDSL, HDSL, and VDSL interfaces,
set top units are increasingly modular, and other interfaces can include Ethernet,
MMDS,coaxial cable, V.34 modem, and ISDN, among others.
The process of sending a transmission over a physical medium for purposes of
Simple Network Management Protocol
The network management protocol used with TCP/IP-based Internets.
SMDS Interface Protocol
Formal name given to each layer of the SMDS network interface.
An ATM physical layer implementation supporting 51 Mbps.
An ATM physical layer implementation supporting 155 Mbps.
Subscriber Network Interface
SMDS term describing generic access to a SMDS network over a dedicated circuit which can
be DS-0, DS-1, or DS-3.
Switched Multimegabit Data Service
A connectionless data service based on global addressing that enables communications
between sites typically at speeds between 1.5 Mbps and 34 Mbps.
Switched Virtual Circuit
A term found in frame relay and ATM networking in which a virtual connection, with
variable endpoints, is established through an ATM network at the time the call is begun;
the SVC is de-established at the conclusion of the call. See also Permanent Virtual
Symmetrical Digital Subscriber Line
A DSL connection that provides equivalent upstream and downstream transmission rates.
Synchronous Digital Hierarchy
The international standard for transmitting digital information over optical networks.
Term used by ITU to refer to SONET.
Synchronous Optical Network
Standards for transmitting digital information over optical networks. Fiber optic
transmission rates range from 51.84 Mbps to 13.22 Gbps. It defines a physical interface,
optical line rates known as Optical Carrier (OC) signals, frame formats, and an OAM&P
(Operations, Administration, Maintenance, and Provisioning) protocol.The base rate is
known as OC-1 and runs at 51.84 Mbps. Higher rates are a multiple of this such that OC-12
is equal to 622 Mbps (12 times 51.84 Mbps).
Data transmission using synchronization bytes, instead of star/stop bits, to control the
transmission. In xDSL, videostreams are considered to be synchronous in nature. See
Synchronous Transport Module-1
SDH standard for transmission over OC-3 optical fiber at 155.52 Mbps.
Synchronous Transport Signal-1
SONET standard for transmission over OC-1optical fiber at 51.84 Mbps.
Systems Network Architecture
IBM's vendor-specific, connection-oriented virtual circuit network architecture for
The US equivalent of E-1. This is a Bell system term for a digital carrier facility used
for transmission of data through the telephone hierarchy at a transmission rate of 1.544
The US equivalent of an E-3. This is a Bell system term for adigital carrier facility used
for transmission of data through the telephone hierarchy at a transmission rate of 45
A generic term for the local telephone company operator in a given area. In the US, the
major telcos are the seven regional Bell operating companies and the leading independent
telcos, GTE, SNET, and Sprint; in Europe, Asia, and elsewhere, the term "telco"
generally refers to the incumbent monopoly, but increasingly refers to competing local
providers as well.
Telephony over Passive Optical Network
Telephony using a PON as all or part of the transmission system between telephone switch
A program that lets you connect to other computers on the Internet.
Time Division Multiplexing
Technique where data from multiple channels may be allocated bandwidth on a single wire
pair based on time slot assignment.
Transmission Control Protocol/Internet
The protocols (TCP/IP) are the result of the Defense Advanced Research Projects Agency
(DARPA) project to interconnect disparate computer networks of the 1970s. Today the
protocols are the basis of the Internet.
A form of error correction found in many modems that allows for forward error correcting
to account for bit errors from various interference on communications lines, such as cross
talk and background noise.
A technology that enables one network to send its data via another network's connections.
Tunneling works by encapsulating a network protocol within packets carried by the second
network. For example, Microsoft's PPTP technology enables organizations to use the
Internet to transmit data across a VPN. It does this by embedding its own network protocol
within the TCP/IP packets carried by the Internet.
A common form of copper cabling used for telephony and data communications. It consists of
two copper lines twisted around each other; the twisting protects the communications from
electromagnetic frequency and radio frequency interference. See also Unshielded Twisted
Uniform Resource Locator
Used with the World Wide Web as an address.
Also referred to as G.lite, UADSL is focused on providing a mass-market version of ADSL,
which is interoperable with full rate ADSL, but with fewer complexities and less overall
requirements at a tradeoff for speed. The solution is intended to reduce the need for a
"splitter" box installed outside the home or new wiring in the home. UADSL
enables plug-and-play and PC-integrated solutions.
Universal ADSL Working Group
The UAWG is composed of leading PC industry, networking, and telecommunications companies
and was established to develop a set of contributions building on the T1.413 standard
intended to create quick deployment and adoption of Universal ADSL (G.lite).
Unshielded Twisted Pair
A cable with one or more twisted copper wires bound in a plastic sheath. Preferred method
to transport data and voice to business workstations and telephones. Unshielded wire is
preferred for transporting high-speed data because at higher speeds radiation is created.
If shielded cabling is used, the radiation is not released and creates interference.
Unspecified Bit Rate
Does not guarantee any throughput levels. This isused for applications, such as file
transfer, that can tolerate delays.
In xDSL, the communications from the customer site up into the telecommunications network.
Variable Bit Rate
Provides a specified throughput capacity but data is not sent evenly. This is a popular
choice for voice and videoconferencing data.
Very High Data Rate DSL
Modem for twisted pair access operating at data rates from 12.9 to 52.8 Mbps with
corresponding maximum reach ranging from 4,500 feet to 1,000 feet of 24-gauge twisted
Video on Demand
The ability to activate a stored or live motion picture stream; in xDSL the application
that allows subscribers to view movies or other video programming on request, similar to
cable television's Pay-Per-View. See Pay-Per-View.
The ability to view real-time video communications on a two-way or multipoint basis. Also
Virtual Channel Identifier
The address or label of a virtual circuit. As an ATM term, it is a unique numerical tag as
defined by a 16-bit field in the ATM cell header that identifies a virtual channel, over
which the cell is to travel.
A logical connection or packet-switching mechanism established between two devices at the
start of transmission.
Workstations connected to an intelligent device which provides capabilities to define LAN
Virtual Path Identifier
As an ATM term, it is an eight-bit field in the ATM cell header which indicates the
virtual path over which the cell should be routed.
Virtual Private Network
A network that is constructed by using public wires to connect nodes. For example, a
number of systems enable creation of networks using the Internet as the medium for
transporting data. These systems use encryption and other security mechanisms to ensure
that only authorized users can access the network and that the data cannot be intercepted.
In telephony, typically the range is from zero to four KiloHertz.
Wide Area Network
Private network facilities, usually offered by public telephone companies but increasingly
available from alternative access providers (sometimes called Competitive Access
Providers, or CAPs), that link business network nodes.
A generic term for the suite of DSL services, where the "x" can be replaced with
any of a number of letters, including "A," "H," "M,"
"RA,""S," and "V." See also Asymmetrical DSL, High Bit Rate
DSL, Moderate Speed DSL, Rate Adaptive DSL, Symmetric DSL, and Very High Data Rate DSL.